1. Field of the Invention
The invention relates to the production of strip steel and, more particularly, to a technique for continuously casting strip steel.
2. Description of the Prior Art
Early steel making processes produced steel slabs in batches. A quantity of liquid steel was poured into individual cast iron molds where solidification into ingots occurred. After cropping and surface conditioning treatments to remove inclusions and surface imperfections, the ingots were rolled into slabs which thereafter were rolled into sheets or strips. As used herein, the terms "strip" or "strip steel" will refer to any finished, or semi-finished, or near-finished steel product having a rectangular cross-section with a width substantially greater than its thickness. Finished strip steel typically has a thickness of 0.015-0.150 inch.
A problem with batch, or ingot, casting is that the steel is poured through air into the molds. Unfortunately, when liquid steel is contacted with air, it is highly vulnerable to re-oxidation, a condition which produces large quantities of solid oxide inclusions. Although most of the inclusions float to the top of the ingot as the ingot solidifies, it still is necessary to crop 10-20 percent from the top of the ingot in order to remove the inclusions. Surface conditioning removes another 1-4 percent of the ingot. The overall slab-producing process is slow and costly because the slabs are produced from individual ingots and because losses are high.
Techniques recently have been developed for continuously casting steel slabs. These techniques also substantially eliminate re-oxidation, thereby greatly increasing the quality of the as-cast slabs. In a typical continuous casting operation, liquid steel is poured through air into a reservoir known as a tundish where inclusions float to the surface. The liquid steel is fed by gravity from the bottom of the tundish through a ceramic nozzle into a cooled, vertically extending, open-ended mold. The steel is backed up in the mold so that the lower end of the nozzle always is immersed in liquid steel, thereby preventing re-oxidation. Upon oscillating the mold, steel being solidified in the mold will be withdrawn from the bottom of the mold. By appropriate control of process parameters such as pour rate, rate of heat removal, and oscillation rate, slabs can be cast continuously at 30-80 inches per minute. The resultant slabs are of very high quality.
Despite the significant advances made by continuous casting processes, several problems have not been addressed adequately. One of the problems relates to the expense of the equipment needed to process the slabs. The capital costs for a rolling mill to convert the slabs into strip is on the order of $350-500 million (1984-85 dollars). The expense is so great because it is difficult to reduce the relatively thick slabs to thin strips. Unfortunately, the slabs cannot be made thinner than about 6-10 inches. This is because the nozzle must extend into the mold so that re-oxidation is prevented. A nozzle typically has a minimum outer diameter of about 4.0 to 5.0 inches. It has been found impractical to produce nozzle-accommodating molds having a thickness less than about 6-10 inches.
The practical consequence of producing a slab having a thickness of about 6-10 inches is that the slab must be reduced to about 1/100 of its original thickness to form strip. The expense of the equipment needed to perform such a large reduction is significant. Accordingly, although high quality slabs can be produced at high production rates by a continuous caster, the expense of the equipment needed to handle the slabs is greater than ever.
A recent attempt to deal with the noted problems is the "thin slab" caster of Con-Cast of Montvale, N.J. In this caster, the upper end of the mold is configured to more closely approximate the external configuration of the nozzle. The upper end of the mold also is tapered to produce a slab having a thickness of approximately 11/2-2 inches. The capital costs of the rolling mill equipment needed to handle a slab produced by the thin slab process is greatly reduced, on the order of $100 million. Also, production speeds are improved, on the order of 140-180 inches per minute. However, the thin slab process suffers from a drawback common to all gravity-fed casting processes wherein a "breakout," or loss of liquid steel, can occur in the event of a mold rupture or other equipment failure.
Another attempt to deal with the noted problems is the so-called horizontal caster. In this approach, the mold is connected to an opening in the side of the tundish by means of a refractory joint known as a break ring. Liquid steel is fed by gravity from the tundish into the mold. Unfortunately, due to the break ring connection between the tundish and the mold, the mold cannot be oscillated independently of the tundish. Also, if flow problems occur in the mold or in the break ring, the entire tundish must be drained. Horizontal casting also is subject to break out problems, as well as to problems related to maintaining surface quality and uniformity of cross-section in the resultant slab. The horizontal casting technique has been found to be suitable only for the production of billets (having a square or near-square cross-section) rather than thin slabs or sheets.
It also is known to cast metal in an upward direction. One early attempt is disclosed in U.S. Pat. No. 2,553,921 issued May 22, 1951 to J. F. Jordan. Jordan discloses a water-cooled, metallic "mold pipe" having an outer ceramic covering that is immersed in a melt. A starter bar is inserted into the mold prior to the beginning of a casting operation. The starter bar is withdrawn from the mold in order to pull solidified metal from the melt. Jordan does not address various problems, such as how to arrange the ceramic covering relative to the mold pipe in order to successfully cast steel. Jordan also fails to address such problems as how to efficiently cool the mold and how to direct the newly cast metal into equipment positioned downstream of the mold.
Other up-casting techniques are disclosed in U.S. Pat. No. 3,746,077 issued May 12, 1971 to T. J. J. Lohikoski, et al. and U.S. Pat. No. 3,872,913, issued Mar. 25, 1975 to T. J. J. Lohikoski. In the '913 patent, the problems associated with thermal expansion differences are avoided by placing only the tip of a "nozzle" in the melt. A water-cooled jacket encloses the upper end of the nozzle. Because the surface of the melt is below the cooling zone, a vacuum chamber at the upper end of the nozzle is necessary to draw the melt upwardly to the cooling zone. The presence of the vacuum chamber limits the rate of strand withdrawl and requires a seal.
The '077 patent avoids the vacuum chamber by immersing a cooling jacket and a portion of an enclosed nozzle into the melt. The immersion depth is sufficient to feed melt to the solidification zone, but it is not deeply immersed. The jacket, as well as the interface between the jacket and the nozzle, are protected against the melt by a surrounding insulating lining. The lower end of the lining abuts the lower outer surface of the nozzle to block a direct flow of the melt to the cooling jacket. Both the '913 and the '077 patents suffer from the drawback that a gap exists between the inner surface of the mold and the outer surface of the newly cast metal. Accordingly, the metal is cooled inefficiently by means of radiation cooling.
The foregoing systems commonly are characterized as "closed" molds in that the liquid metal communicates directly with the solidification front. The cooled mold typically is fed from an adjoining container filled with the melt. In contrast, an "open" mold system feeds the melt, typically by a delivery tube, directly to a mold where it is cooled very rapidly. Open mold systems commonly are used in downcasting large billets of steel and occasionally aluminum, copper, or brass. However, open mold casting is not used to form products with a small cross-section because it is very difficult to control the liquid level and hence the location of the solidification front. Also, if steel is being cast, many steel grades require the total absence of air during mold entry. It is very difficult to protect the entry stream of molten steel from contacting air during continuous pouring into open molds of small cross-section.
Yet additional approaches to the upcasting of metal are known. These additional techniques primarily are directed to copper and brass castings. The references in question are U.S. Pat. No. 4,232,727, issued Nov. 11, 1980 to Terry F. Bower, et al., U.S. Pat. No. 4,307,770, issued Dec. 29, 1981 to George Shinopulos, et al., and U.S. Pat. No. 4,612,971, issued Sept. 23, 1986 to Terry F. Bower, et al. The '727 patent describes a method and apparatus for the continuous production of strip from a cast rod in which the rod is drawn through a mold from a melt in the pattern of forward and reverse strokes. The '770 patent is directed to a particular mold assembly for upcasting strands of copper alloys such as brass wherein a refractory insulating means is provided between the mold cooling means and the die. The '971 patent discloses a continuous method for production of metallic strip from a melt which includes regulating the speed of a metallic rod to maintain a substantially constant forward speed before the rod is converted to strip, the regulation being accomplished by (1) changing the direction of travel of the rod after emergence from a chilled mold, (2) permitting slack through lateral deflections of the rod, and (3) advancing the rod in a manner to control the slack.
Since the ultimate product of the process is to be a metal strip having a width substantially greater than its thickness, it would be useful to provide a mold have a near-net shape cross-section. This would greatly reduce the amount of hot rolling required to produce the final strip of material. Since the hot rolling process generally is conducted in a horizontal plane, it would be well if some means could be found to provide a transition from the direction of mold issuance to a horizontal plane.
Most desirably, a technique would be found that would enable strip steel to be cast directly from the tundish, thereby minimizing the expense of the rolling equipment needed to finish the steel. Hopefully the technique would avoid re-oxidation and breakout problems, as well as other problems associated with prior continuous casting techniques.